Pitot tube type flowmeters are commonly used for measuring a flow rate of exhaust gas of an internal combustion engine, for example, as illustrated in Japanese Laid-open Patent Publication No. 2014-20808. The pitot tube type flowmeter includes a dynamic pressure tube that measures a dynamic pressure of exhaust gas flowing through an exhaust gas tube, a static pressure tube that measures a static pressure of exhaust gas flowing through the exhaust gas tube, and a differential pressure gauge that measures a differential pressure between dynamic pressure and static pressure. Further, the pitot tube type flowmeter calculates a current speed on the basis of the differential pressure and multiplies the current speed by a cross-section area of the exhaust gas tube to measure the flow rate. On the dynamic pressure tube, measuring holes for introducing the exhaust gas are provided in a facing direction to a flowing direction of the exhaust gas, and on the static pressure tube, measuring holes are provided in an opposing direction to the flowing direction of the exhaust gas.
From a standpoint of a configuration of the pitot tube type flowmeter, when this dynamic pressure tube is clogged with foreign materials, the dynamic pressure is not correctly measured and the flow rate of the exhaust gas is not correctly measured. To deal with the above, conventionally, irrespective of whether or not the dynamic pressure tube is clogged with foreign materials, the dynamic pressure tube is forcibly purged at each predetermined timing to secure correct operations of the pitot tube type flowmeter.
Also in flow sensors other than the pitot tube type flowmeter, when troubles occur in the flow sensor, an exhaust gas flow rate of the internal combustion engine is not correctly measured. Therefore, it is necessary to detect the troubles in the flow sensor and secure normal operations.
However, with the configuration as described above where the dynamic pressure tube is forcibly purged at each predetermined timing, there is a problem that it is difficult to purge the dynamic pressure tube at appropriate timing, so that the dynamic pressure tube cannot be immediately purged to remove the foreign materials when clogged with foreign materials. In addition, in the case in which the dynamic pressure tube is forcibly purged, there is a problem that, even if not clogged with foreign materials, the dynamic pressure tube is purged, and therefore it is excessively purged.